Metroid Prime 2 possibly. A new "realistic" Zelda is rumored to come out 2004 for GCN. Star Fox Armada will also probably be scheduled for 2004. Don't forget about Animal Crossing 2, whose predecessor was a hit in both Japan and America. Many people are hyping Too Human, a game that literally nobody knows information about besides tidbits. Also, Mario Party 5 might come out 2004, this time with LAN features. The new Final Fantasy will come out 2004 if it is not released this year. Adding to Too Human is Metal Gear Solid, which I think will come out next year. A rumored Pokemon RPG is in the works supposedly, possibly coming out next year. Dead Phoenix and Killer 7 will also come out next year, most likely.
However, you asked for the big game of 2004 Gamecube-wise. That, in my opinion, is the highly anticipated Resident Evil 4. I am hyped for it much more than I was for Metroid Prime or the Wind Waker. Not only that, but it is guaranteed to sell a good amount of Cubes if it is as good as I expect it to be.